This invention relates to printers. More specifically, it relates to printer assemblies that employ linear arrays of symbol generation elements by which the recording medium is passed, particularly thermal printers having heating elements arranged in a linear array on a print head which creates characters or other symbols on thermal printer paper as it passes by the print head.
Thermal printers are well known and widely used for many applications, particularly where high printing speed or small physical space are required. For example, a thermal printer may be particularly suitable for incorporation in a portable medical patient monitoring instrument, where the principle function of the instrument is to measure accurately, display and record physiological waveforms and parameters, as well as trend information about the patient""s vital signs. Thermal printers lend themselves to high printing speeds which are needed to print waveforms, such as an electrocardiogram, in real time so as to be immediately available to clinicians.
At the same time, image quality is always a concern with a printer to ensure that the information recorded thereby can easily and accurately be read. In essentially any printer where, as in a thermal printer, a two dimensional image is created by moving the printing paper between a platen and a linear array of printing elements disposed on an elongate print head forced against the paper, it is important that the force applied by the print head to the paper be substantially uniform throughout the length of the paper. Meeting this requirement can be a challenge because of the need also to provide a mechanism for loading paper into the printer so that it passes between the print head and the platen.
A thermal printer assembly that is particularly effective in providing such uniform force while permitting paper to be loaded therein is disclosed in Mahoney U.S. Pat. No. Re. 35,026 entitled SELF-ALIGNING THERMAL PRINT HEAD AND PAPER LOADING MECHANISM, hereby incorporated by reference in its entirety. The printer assembly of this Mahoney patent employs a lateral self-alignment mechanism which urges the print head against the platen to provide uniform lateral distribution force. However, this assembly does have some important limitations. First, it employs a sliding door for opening the printer to load paper, which requires space behind the printer to open the door. Second, it requires the use of flat, folded printer paper which takes more space and often is not as easy to use as a cylindrical roll of printer paper. Third, the paper can be difficult to install because it must be placed into a deep paper well, partially under the sliding cover. Fourth, because the motor for driving the platen is mounted on the frame of the printer and the platen itself is mounted on the cover, the cover must be carefully aligned with the frame so that a gear on the frame will engage a gear on the cover to operate the platen.
Another approach is to use a cylindrical roll of paper and a cover that is hinged so as to swing, rather than slide, open. A printer assembly which uses this approach is shown, for example, by Gustavsson et al. U.S. Pat. No. 5,791,796, entitled THERMAL PRINTER WITH SPRING-BIASED DRIVE ROLLER/PLATEN. In addition, the Gustavsson et al. printer employs a platen drive motor mounted on the door, which avoids the problem of aligning the door with the frame so as to ensure that gears properly mesh. However, the Gustavsson et al. device does not provide for lateral self-alignment of the print head with the platen so as to apply uniform force against the platen.
Although it would be desirable to employ a cylindrical roll of paper and a door that is hinged so as to swing, rather than slide, open, the application of uniform force is important to achieve good print quality. The problem is that a lateral self-alignment mechanism of the type shown in Mahoney, which has a sliding door, will not readily work with a swinging door of the type shown in Gustavsson et al., at least without undue complexity, because the platen and the print head will interfere with one another when the door is opened and closed.
Accordingly, there is a need for a printer assembly that provides the advantages of lateral self-alignment of a print head with a platen and the advantages of a door that swings open and closed so as to provide better utilization of space, ease of use and high print quality.
The present invention meets the aforementioned need by providing a printer assembly with both lateral and longitudinal self-alignment. The printer assembly has a frame, a top door hinged at the back of the frame to swing open, a motor-driven platen disposed at the front, underside of the door, a print head disposed at the front of the frame, and a spring coupled to the print head so as to produce both backward force to urge the print head against the platen when the door is closed and torque to rotate the print head upwardly when the door is opened. The print head is disposed on a mounting assembly which has a centrally located tab that provides a fulcrum and engages a spring. The spring provides both backward force and torque to rotate the print head upwardly when the door is open.
The platen is driven by a motor which is also mounted on the door and connected by a gear train to the platen so as to transmit rotational motion to the platen. The frame is provided with a semi-cylindrical well for receiving a cylindrical roll of printer paper, and has a generally triangular shape for easy access to the well when the door is open.
Therefore, it is a principal object of the present invention to provide a novel and improved printer assembly with both lateral and longitudinal self-alignment.